Paula73, just use a gluten free flour mixture in place of the wheat flour. I have used a mixture of brown rice and corn starch and it works just fine. Using only starch in it would probably work as well but use less, maybe mix a 1/8c with a bit of milk and add to the soup. Adding more starch that is mixed with milk as necessary to get to the right thickening. If you get too much starch it will be lumpy and like a big glob of soup. I know this from experience. :)
Oh, make certain your broth, if purchased, does not have wheat or gluten in the mix. Many do!